In video compressions standards like MPEG-1, MPEG-2, MPEG-4, or MPEG-4 AVC, a compressed picture is usually divided into rectangle units called “macroblocks”. A macroblock is usually defined as a two-dimensional block of image samples. The image samples have a width of 16 pixels and a height of 16 pixels for luminance samples. The compression ratio for the macroblock is controlled by a quantization scale parameter for each macroblock.
The quantization scale parameter determines the level of quantization to be applied to all the frequency coefficients. The quantization scale parameter is usually coded as a difference value from the quantization scale parameter of the previous macroblock in cording order, and is stored in a compressed macroblock header.
In new video standards under development, for example, the High Efficiency Video Coding (HEVC) standard by the MPEG standardization bodies, it is suggested that dividing the picture into large units can improve the coding efficiency of the compressed video (for example, refer to Non Patent Literature 1). In other words, a picture can be divided into coding units (CU) where each coding unit has a size that can be much larger than a macroblock. For example, the coding unit size can be 128 pixels by 128 pixels for luminance samples, which is approximately 64 times larger than a macroblock.
A large coding unit can be sub-divided into smaller units (sub coding units) to achieve better coding efficiency. Each coding unit or sub coding unit has three main components. The main components are a coding unit header, a prediction unit (PU), and a transform unit (TU).
FIG. 1 is a diagram showing the structure of compressed picture having coding units.
As shown in FIG. 1, a picture D100 includes a header (hereinafter referred to as picture header) and a body. The picture header includes parameters related to the picture (picture parameters) while the body includes compressed samples of a picture. Moreover, the body includes coding units such as coding units D102 and D104, and some of the coding units are divided into sub coding units. For example, the coding unit D102 is divided into sub coding units D106, and one of the sub coding units 106 is further divided into smaller sub coding units D108. The coding unit D104 or sub coding unit D108 has three main components. More specifically, the coding unit D104 includes a coding unit header D116, a prediction unit D118, and a transform unit D120 as the three main components. The sub coding unit D108 has a sub coding unit header D110, a prediction unit D112, and a transform unit D114 as the three main components. As shown in FIG. 1, a transform unit D120 is divided into small sub transform units D122, and one of the sub transform units D122 is divided into smaller sub transform units D124. The smallest transform units (sub transform units) D114 and D124 includes the quantized coefficients of a block, which requires a quantization scale parameter for the inverse quantization process of the coefficients.